In a color image forming apparatus as one type of image forming apparatus for obtaining a multiple color or full color image, mixed toner images of the three primary colors (yellow, magenta, cyan) are required to be formed by being superposed on a recording sheet.
In such color image forming apparatus for reproducing a multiple color image, color toners representing the three primary colors are generally used to sequentially form the respective color images which will then be superposed in turn so as to form one image of multiple colors or full-color.
As an example of such a color image forming apparatus, an image forming apparatus in which a color image can be reproduced by forming a number of toner images of respective colors and by sequentially transferring, on a one-by-one basis, each color toner image onto recording sheet as disclosed in Japanese Laid-Open Patent Application 52-73738/1977 is known.
The aforementioned color image forming apparatus includes a clamp mechanism for clamping the trailing edge of the recording sheet, so that the recording sheet is held in contact with a photoconductive element during a number of image forming operations for a number of different colors. However, the clamp mechanism is extremely complex and the image cannot be transferred to the area of the recording sheet to be clamped.
To solve the above problems, a color image forming apparatus is proposed with an intermediate transfer element, and the toner images of each of the color components are first formed on the photoconductive element in a number of operations. Then, the color toner image is formed by transferring the toner image of each of the color components superposed onto the intermediate transfer element in turn, and the color toner image is transferred onto the recording sheet.
The aforementioned intermediate transfer element is, after the carried image is transferred onto the recording sheet, prepared for the next transferring operation of the toner image by scraping off the toner remaining on the surface of the intermediate transfer element by using a cleaning blade or the like.
In the image forming apparatus as proposed above, there are a number of advantages such as increased spatial flexibility of the location for the intermediate transfer element by using a belt shaped member which is movably positioned between two rollers (a drive roller and a driven roller) as an intermediate transfer element, and reduced size of the image forming apparatus.
As a driving power source for moving the intermediate transfer element, a stepping motor or the like driving power source is often used so that the image transferring position of the intermediate transfer element may be easily controlled, being separate from a driving power source of the photoconductive element.
Furthermore, in a color image forming apparatus in which the aforementioned intermediate transfer element is used, it is required to be available for mostly all kinds of image forming sizes. Therefore, the size of the intermediate transfer element, particularly, the circumferential length corresponding to the longitudinal direction of the original document to be reproduced, often tends to be made longer.
However, when the circumferential length of the intermediate transfer element is determined according to the maximum image forming size, and if the image forming operation is executed on the basis of the original document size which is smaller than the aforementioned image forming Size, the image transferring area in the image transfer element is smaller than the maximum image forming area. Therefore, the required time for executing a copying operation for obtaining a copy is made relatively long, whereas the image transferring area is relatively small, when the moving speed of the intermediate transfer element is based on the largest size of the image forming area. Furthermore, in the case of mounting a driving power source for both the photoconductive element and the intermediate transfer element, the structure of the image forming apparatus is made relatively complex and relatively large.
Presently, there is no such image forming apparatus which solves the above-mentioned problems of complexity and size.